On the spin-isospin response at intermediate energies

We have calculated the response function of infinite nuclear matter to a spin-isospin probe in the transverse and longitudinal channels. The two channels are distinguished by the meson exchanged (rho or π, respectively) in the ph and Δh interactions, in addition to the effective g' interaction. In the intermediate-energy region the response is particularly sensitive to the Δ-dynamics in the nuclear medium. We have applied the results of a previous calculation, which is based on a self-consistent treatment of π- and Δ-propagation in nuclear matter, to obtain Δ-self-energy corrections in the medium. Due to the opening of many-body decay channels, the self-consistent mechanism supplies a strong quenching and spreading effect on the Δ-peak in the transverse channel and we find a remarkable contribution to the response function in the energy region corresponding to the dip in inelastic electron scattering. A comparison with the experimental data for the 12C(e, e') cross section is given. In the longitudinal channel, on the other hand, the coherent propagation of the intermediate pion has the effect of splitting the Δ-peak into two parts, due to the vanishing of the polarization propagator at the energy corresponding to the on-mass-shell pion. The self-consistent calculation again yields a quenching effect on the response which is particularly strong on the lower peak and pushes up the other one to higher energies. (orig.)